Builder Pattern in Effective Java

Question

I have recently started to read Effective Java by Joshua Bloch  I found the idea of the Builder pattern  Item 2 in the book  really interesting  I tried to implement it in my project but there were compilation errors  Following is in essence what I was trying to do   The class with multiple attributes and its builder class   public class NutritionalFacts       private int sodium      private int fat      private int carbo       public class Builder           private int sodium          private int fat          private int carbo           public Builder int s                this sodium   s                     public Builder fat int f                this fat   f              return this                     public Builder carbo int c                this carbo   c              return this                     public NutritionalFacts build                 return new NutritionalFacts this                        private NutritionalFacts Builder b            this sodium   b sodium          this fat   b fat          this carbo   b carbo            Class where I try to use the above class   public class Main       public static void main String args              NutritionalFacts n                new NutritionalFacts Builder 10  carbo 23  fat 1  build              I am getting the following compiler error      an enclosing instance that contains   effectivejava BuilderPattern NutritionalFacts Builder   is required                   NutritionalFacts n   new   NutritionalFacts Builder 10  carbo 23  fat 1  build      I do not understand what the message means  Please explain  The above code is similar to the example suggested by Bloch in his book

User · Answer

You need to declare the Builder inner class as static    Consult some documentation for both non-static inner classes and static inner classes   Basically the non-static inner classes instances cannot exist without attached outer class instance

User · Answer

To generate an inner builder in Intellij IDEA  check out this plugin  https   github com analytically innerbuilder

User · Answer

You are trying access a non-static class in a static way  Change Builder to static class Builder and it should work   The example usage you give fails because there is no instance of Builder present  A static class for all practical purposes is always instantiated  If you don t make it static  you d need to say   Widget   new Widget Builder 10  setparm1 1  setparm2 3  build      Because you would need to construct a new Builder every time

User · Answer

The Builder class should be static   I don t have time right now to actually test the code beyond that  but if it doesn t work let me know and I ll take another look

User · Answer

Make the builder a static class  Then it will work  If it is non-static  it would require an instance of its owning class - and the point is not to have an instance of it  and even to forbid making instances without the builder   public class NutritionFacts       public static class Builder             Reference  Nested classes

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Once you ve got an idea  in practice  you may find lombok s  Builder much more convenient    Builder lets you automatically produce the code required to have your class be instantiable with code such as   Person builder      name  Adam Savage      city  San Francisco      job  Mythbusters      job  Unchained Reaction     build       Official documentation  https   www projectlombok org features Builder

User · Answer

You need to change Builder class to static class Builder  Then it will work fine

User · Answer

This mean that you cant create enclose type  This mean that first you have to cerate a instance of  parent  class and then from this instance you can create nested class instances    NutritionalFacts n   new NutritionalFacts    Builder b   new n Builder 10  carbo 23  fat 1  build      Nested Classes

User · Answer

I personally prefer to use the other approach  when you have 2 different classes  So you don t need any static class  This is basically to avoid write Class Builder when you has to create a new instance   public class Person       private String attr1      private String attr2      private String attr3          package access     Person PersonBuilder builder            this attr1   builder getAttr1                                            getters and setters     public class PersonBuilder       private String attr1      private String attr2      private String attr3          constructor with required attribute     public PersonBuilder String attr1            this attr1   attr1             public PersonBuilder setAttr2 String attr2            this attr2   attr2          return this             public PersonBuilder setAttr3 String attr3            this attr3   attr3          return this             public Person build             return new Person this                         So  you can use your builder like this   Person person   new PersonBuilder  attr1                                setAttr2  attr2                                build

User · Answer

As many already stated here you need to make the class static  Just small addition - if you want  there is a bit different way without static one    Consider this  Implementing a builder by declaring something like withProperty value  type setters inside the class and make them return a reference to itself  In this approach  you have a single and an elegant class which is a thread safe and concise   Consider this   public class DataObject        private String first      private String second      private String third       public String getFirst           return first              public void setFirst String first          this first   first                        public DataObject withFirst String first          this first   first         return this              public DataObject withSecond String second          this second   second         return this              public DataObject withThird String third          this third   third         return this             DataObject dataObject   new DataObject         withFirst  first data         withSecond  second data         withThird  third data      Check it out for more Java Builder examples

User · Answer

You should make the Builder class as static and also you should make the fields final and have getters to get those values  Don t provide setters to those values  In this way your class will be perfectly immutable   public class NutritionalFacts       private final int sodium      private final int fat      private final int carbo       public int getSodium            return sodium             public int getFat            return fat             public int getCarbo            return carbo             public static class Builder           private int sodium          private int fat          private int carbo           public Builder sodium int s                this sodium   s              return this                     public Builder fat int f                this fat   f              return this                     public Builder carbo int c                this carbo   c              return this                     public NutritionalFacts build                 return new NutritionalFacts this                        private NutritionalFacts Builder b            this sodium   b sodium          this fat   b fat          this carbo   b carbo            And now you can set the properties as follows   NutritionalFacts n   new NutritionalFacts Builder   sodium 10  carbo 15   fat 5  build

[java] Builder Pattern in Effective Java

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